Descripción del experimento (programa)

Whilst orbital and spin periods have been detected in a large number of SFXTs there are also a number of sources in which this is not the case. In the case of systems with unidentified orbital periods this is most likely due to the system properties providing challenges to the identification of the binary orbit. In some cases it is simply the lack of diagnostic information provided by the small number of detected outbursts that prevents the determination of the orbital period (e.g.

IGR J20188+3647 and IGR J21117+3427; Sguera et al. 2006b). Such a small number of outbursts may be the result of the systems possessing long, wide orbits such that outbursts are rarely generated or they may be located at large distances so that only the most luminous outbursts are detectable. In other cases systems can display much higher levels of outburst activity without a periodicity being

detectable (e.g. IGR J18410−0535 which has a duty cycle of inactivity of only 28%; Romano et al. 2009b). In these cases, where the sources also have confirmed stellar counterparts confirming the binary nature of the X-ray source, the orbits are likely near-circular such that there is no enhancement of emission during a preferential orbital phase to allow the identification of the orbital period. The systems should also be at low inclinations so that X-ray eclipses are not produced to provide a dynamical signal. It may be, however, that with a continued increase in exposure, the orbital periods of the non-periodic SFXTs will be revealed. In the case of spin periods it is the pulsating SFXTs that are the minority with many of the well studied systems yet to have a pulse period identified. The origins of this lack of spin period detections are likely varied but, again, observational limitations provide obstacles to the identification of the periodic signals. With peak luminosities of ∼1036_{erg s}−1 _{SFXTs are detected by hard X-ray monitors with insufficient}

signal-to-noise to allow the identification of pulsations in this band. At softer energies many observations are short, snap-shot exposures (a few ks) which have an insufficient duration to allow the detection of signals in the range of hundreds to thousands of seconds. Additionally, during longer observations with more sensitive instruments, the pulsed signal can be masked by the large scale flaring that occurs on similar, ks timescales during active states or the generation of the pulsed signal may be inhibited during observations of SFXTs in the quiescent state if magnetic or centrifugal barriers prevent matter from entering the magnetosphere and forming accretion columns. Obscuring material close to the NS may also act to disrupt and dampen the pulsed signal in these (sometimes) highly absorbed systems. Hence the detection of pulsations from the SFXTs is challenging and, along with the fact that the NSs in some systems may be inappropriately aligned to produce observable pulsations, helps to explain why the number of SFXTs with firmly identified NS spin periods is rather small.

The spread of the SFXTs that do have identified orbital or spin periods in the Corbet parameter space (Fig. 6.1), however, is intriguing. Given that the orbital periods of SgXRBs would not be expected to change significantly during the wind-fed X-ray binary stage of their evolution, due to the relatively low mass-loss rate of the supergiant (< 10−6Myr−1) compared to the total mass of the binary

during the short lived (< Myr) supergiant phase, and the pulse periods would also remain fairly constant due to the inefficient, randomly orientated transfer of angular momentum provided through wind accretion, this implies that SgXRBs occupy their current locations in the Corbet diagram at the onset of the X-ray binary

phase. Hence the SFXTs would also have occupied their current locations at the start of the X-ray binary phase, locations which appear consistent with the nominal SgXRB evolutionary track for the shorter orbital period systems but are distinct in the longer period cases. Therefore certain members of the SFXT class may be illustrating an evolutionary pathway to the SgXRB phase that is not revealed in the classical SgXRB population. Liu, Chaty, & Yan (2011) also considered this

implication on the evolutionary pathways of HMXBs using the specific examples of IGR J18483−0311 and IGR J11215−5952. In this work, which is briefly

summarised and expanded upon here, the authors considered four scenarios under which the spin periods of these SFXTs could have reached their current values through the considerations of the equilibrium spin periods of the NSs in these binaries. The equilibrium spin period of a NS in a binary (Corbet, 1984) is the spin period at which the outer edge of the magnetosphere rotates with the same velocity as the local Keplerian velocity (i.e. RM= Rco in the notation used previously in this

work). As the spin period of a NS in a binary evolves after the SN explosion of its birth, it tends towards its equilibrium spin period as at this point the ‘spin-down’ induced by the expulsion of matter that is centrifugally inhibited from accreting at short rotation periods is balanced by the ‘spin-up’ induced by accretion of matter on to the NS surface at long rotation periods. In the classical SgXRBs, however, it is believed that NSs possess the equilibrium spin periods corresponding to when their companion stars were still residing on the main sequence (Waters & van Kerkwijk, 1989). This results from the fact that the stellar mass loss rates required to achieve a quantitative agreement between the theoretical and observed NS spin periods in SgXRBs are two orders of magnitude below those expected from supergiant stars. The SFXTs IGR J18483−0311 and IGR J11215−5952, however, have

comparatively short spin periods, for their orbital periods, compared to the

equilibrium spin period expected to result from the interaction with the companions stellar wind during the MS phase. Liu, Chaty, & Yan (2011) investigated the possible origins of these comparatively short spin periods. They excluded the possibilities that these SFXTs reached their current locations because:

1. they are in fact spinning at the MS equilibrium periods as this requires the NSs to possess a B-field of only ∼1011G, for which there is no observational evidence within the HMXB population;

2. they have been spun-up from their equilibrium spin periods to their current location due to accretion in the SgXRB phase as the randomly orientated, inefficient transfer of angular momentum prevents coherent spin-up in non-RLO SgXRBs;

3. that the NSs did not reach their equilibrium spin periods before the

companion evolved off the MS due to the incompatibility of NS B-field decay timescales for nominal natal fields compared to the MS lifetime of the

companion star.

Instead these authors argue that IGR J18483−0311 and IGR J11215−5952 are the descendants of OeXRBs in which accretion has been reinitialised after the

companion has evolved off the MS into the supergiant phase. As a result the NSs in these systems would have possessed faster equilibrium spin periods generated by the interaction with the slower, dense equatorial wind of the Oe-star. The NSs would have then spun-down to the equilibrium spin period of the supergiant wind to ensure that the centrifugal barrier was overcome and accretion was re-initiated in the supergiant phase. To test this conclusion the theoretical equilibrium spin period, as a function of orbital period, was calculated for a B1 Ia supergiant using the methods of Waters & van Kerkwijk (1989), and both IGR J18483−0311 and IGR J11215−5952 were observed to be in good agreement with this model as shown in Fig. 6.1.

The theoretical equilibrium spin period curve for NSs interacting with the stellar wind of a supergiant companion has been reproduced and superimposed on the Corbet diagram in Fig. 6.1 using the same assumptions as those of Liu, Chaty, & Yan (2011) (B = 3×1012G, MSg= 20M, RSg= 30 R, ˙MSg= 10−6Myr−1 and

v∞= 2.65vesc). In addition to the agreement of IGR J18483−0311 and IGR

J11215−5952 with this model, it is also of note that IGR J16465−4507 is in approximate agreement with this model under the general system parameters assumed. Furthermore several of the SFXTs without currently identified pulse periods could also be consistent with the OeXRB evolutionary track. Under this interpretation, XTE J1739−302 and SAX J1818.6−1703 would be expected to possess pulse periods in the range of ∼80 − 200 s, and the shorter orbital period systems (IGR J16328−4726, IGR J17354−3255 and IGR J18450−0435) in the range ∼10 − 20 s. Conversely, due to the turnover observed in the theoretical curve, IGR J16479−4514 cannot be consistent with this model as the intersection with its short, 3.32 day orbital period occurs at a pulse period consistent with the initiation of RLO which, given the earlier arguments against RLO, suggests this source is of a slowly rotating nature. Finally it is of note that the classical SgXRB OAO

1657−415 is also close to this curve, which may suggest that the Oe evolutionary pathway was in fact present in the original class of SgXRBs. The location of OAO 1657−415 should be treated with caution, however, as this system is comprised of a NS and an evolved Ofpe/WN9 star that has undergone significant mass-loss, such that the binary has possibly followed an unusual evolution whilst the primary was in the Luminous Blue Variable state (see Mason et al. 2012 for further discussion). The agreement in the locations of some SFXTs in the Corbet parameter space to the theoretical equilibrium spin periods generated by the interaction with the stellar wind of a supergiant, as opposed to MS, companion strongly implies that these systems have evolved directly from OeXRBs. The existence of an evolutionary

link between the BeXRB and SgXRB phase has implications for both the individual systems and the nature of HMXBs as a whole, revealing a separate evolutionary branch for the production of SgXRBs and an extension of the duration of the X-ray emitting phase in the lifetime of massive binary progenitor systems that should be taken into account in population synthesis models. Hence it may be considered that SgXRBs can be separated into two distinct sub-groups depending on whether or not the system has an accretion history prior to the SgXRB phase. The question may be asked, however, as to why such an evolutionary link may have been identified in the SFXTs, a small population of sources that have been discovered only recently. This can start to be understood in terms of an observational selection effect. When the systems were OeXRBs they were likely in long, eccentric orbits as is observed in the current BeXRB population. Hence, when the companion star evolved off the MS, the resulting system would be a NS in a long eccentric orbit, that may have contracted by up to 10% (Liu, Chaty, & Yan, 2011), about a supergiant companion. As such the resulting X-ray source would not be expected to be a classical,

persistent wind-fed SgXRB as the low ambient wind densities at the large apastron separations and the effect of the structure of the supergiant stellar wind, as

discussed in previous chapters and the following paragraphs, would produce a transient X-ray source. Finally, only with the launch of X-ray missions with wide FOVs, high hard X-ray instantaneous sensitivity and a Galactic Plane orientated observing strategy, such as Beppo-SAX /WFC (Boella et al. 1997, Jager et al. 1997) and currently INTEGRAL/IBIS, would such fast transient sources (i.e. SFXTs) be detectable in sufficient numbers to allow the characterisation of their orbital

properties and the identification of such an evolutionary pathway in the population. Whilst the locations of IGR J16465−4507, IGR J18483−0311 and IGR

J11215−5952, along with the possible locations of other members of the SFXT class, within the Corbet diagram provide intriguing insights into the varied evolutionary mechanisms that may form SgXRBs, the detection of further pulse periods is vital in assessing this theory to a higher degree of scrutiny. Independent constraints on the orbital and stellar parameters of individual systems will also allow theoretical evolutionary calculations to be performed within a restricted parameter space that would provide a more thorough testing of this hypothesis on a system by system basis.